1. Field of the Invention
The present invention relates to injection molding machines and, more particularly, to an injection unit of the type which has a plastification cylinder and a cooperating plastification screw which, after plastification of a charge, is forcibly advanced inside the plastification cylinder, thereby injecting the charge of plastic raw material into an injection molding die.
2. Description of the Prior Art
Injection units of the type under consideration commonly have two parallel tie rods of which one extremity is attached to the die closing unit of the injection molding machine and the opposite extremity is free or supported on the machine base. The two tie rods not only serve as a support for the injection unit proper, but they may also double as piston rods for hydraulic actuators which advance and retract the plastification cylinder and/or hydraulic actuators which produce the injection stroke of the plastification screw.
At the end of the injection stroke, it is desirable to achieve a predetermined precise gap between the interior entry surface of the injector head of the plastification cylinder and a matching surface of a guided head portion on the forward extremity of the plastification screw. It is known that there exists an optimal gap size at which the ejector head and the guided head portion of the plastification screw are "self-cleaning," meaning that they will not develop deposits of raw material residue. This optimal or "self-cleaning" gap necessitates the adjustment of a precise optimal injection end position for each injection unit. On the other hand, the determination of this end position requires considerable experimentation, because it is influenced by a number of factors of which some are difficult to control: machining tolerances, dimensional changes resulting from heat treatment, uneven heat expansion due to temperature differences, etc. It follows that the optimal injection end position can only be approximated.
Adding to the difficulty of achieving such an optimal injection end position are recent developments in the field of injection molding technology which involves the processing of very abrasive plastic raw materials. In order to accommodate these materials, it has become necessary to improve the hardness and abrasion resistance of the plastification cylinders and plastification screws, with the result that allowance has to be made for wider dimensional discrepancies of these parts. Aside from these larger tolerances, it has become necessary to utilize longer plastification cylinders and plastification screws, especially in situations where the plastification cylinder is designed to allow for outgassing of the plastic raw materials. The longer units, in turn, require even larger tolerances and they are also more susceptible to heat expansion.
Lastly, it is an important consideration to minimize, or, whenever possible, to eliminate any reworking of the plastification elements after heat treatment, especially when the required physical characteristics necessitate surface treatment of the work pieces. Obviously, the reworking of the surface-treated plastification elements quickly reduces or eliminates their hardened surface layer, thereby correspondingly lowering the longevity of the parts.
In the face of these diverse influences which may account for a deviation of the injection end position from the earlier-mentioned optimal end position, it has already been suggested to provide an axial adjustability of the plastification cylinder relative to its carrier bridge, in order to make it possible to adjust a "self-cleaning" gap between the forward end position of the plastification screw and the injector head. These known adjustments involve the use of axial shims between the plastification cylinder and its carrier bridge.
The known position adjustment procedure of the plastification cylinder by means of axial shims is very time-consuming, as it becomes necessary to repeatedly remove the plastification cylinder, in order to determine empirically the adequacy of the end result of successive position adjustments on the plastification cylinder, in terms of their effect on the injection end position of the plastification screw. And, because it is virtually impossible to achieve a really precise optimal injection end position in this manner, and, because it is time-consuming even to achieve an approximation of this position, it is common practice to accept comparatively coarse approximations as the final adjustment.
The prior art in this field includes known adjustment devices for the continuous adjustment and clamping of tie rods. One such device is disclosed in U.S. Pat. No. 1,080,144 which suggests a threaded end portion on each tie rod carrying a threaded abutment collar in engagement with a pressure face of the plate to which the tie rod is to be attached. Traction bolts pull a clamping cap or similar clamping member against the extremity of the tie rod, thereby axially preloading the latter against the plate. The axial adjustability of the abutment collar on its tie rod is not an essential part of these clamping devices, being incidental in some of them and eliminated through position-setting abutment shoulders in others.
Another adjustment mechanism is disclosed in U.S. Pat. No. 4,105,390 which suggests a mechanism for the axial resetting of the distance between the stationary die carrier plate and the hydraulic actuator of an injection molding machine by means of clampable adjustment mechanisms which are associated with multiple tie rods and operated simultaneously by means of a connecting drive. Each adjustment mechanism includes a threaded adjustment sleeve on a threaded tie rod length portion. The clamping action is obtained by means of a clamping flange and clamping bolts which create an axial preload between the tie rod and the associated plate of the injection unit, via the adjustment sleeve.
Both above-described mechanisms are unsuitable for the longitudinal adjustment of the plastification cylinder of an injection unit, because they cannot accommodate the axially movable plastification screw and/or a single-thread clamping action.